Defining the thumbnails (Album Art)

The thumbnails used in the widget are taken from the posts' featured images. In the event that a podcast does not have a featured image available, Message Flow will check the following locations for a suitable fallback image:

A URL to the fallback image in the shortcode: [message-flow fallback_image="http://example.com/default_image.png"]

If you're using the PowerPress podcasting plugin, the default image will be taken from the iTunes image. PowerPress users can configure this in the "iTunes Feed Settings" section of PowerPress' configuration. (/wp-admin/admin.php?page=powerpress/powerpressadmin_basic.php)

In your theme's directory. Just make sure it's 165 pixels square and named "unknown-album_165.png"